I lost the O-ring from mine as well and have replaced it with one from the hardware store cant remember the size but I just took the airbrush with me and bought a dozen.
I can't believe that someone asking a simple question has to put up with people saying they should invest in a new decent AB, the Aztek vs the rest argument has its place but give it a rest in this sort of post, its not big and its not clever.
I love my aztek and I love my new iwata I just use them for different jobs and as far as learning the tecniques the Aztek did a brilliant job for me.